1. Field of the Invention
The present invention relates to a method of producing a photoelectric conversion device, which is used in solar batteries, CCD sensors, etc.
2. Description of the Related Art
Photoelectric conversion devices, which include a photoelectric conversion layer and electrodes electrically connected with the photoelectric conversion layer, are used in applications, such as solar batteries. The main stream of conventional solar batteries has been Si solar batteries, which use bulk single-crystal Si or polycrystal Si, or thin-film amorphous Si. On the other hand, compound semiconductor solar batteries, which do not depend on Si, are now being researched and developed. As the compound semiconductor solar batteries, those of a bulk type, such as GaAs solar batteries, etc., and those of a thin-film type, such as CIS or CIGS solar batteries, which contain a group Ib element, a group IIIb element and a group VIb element, are known. CI(G)S is a compound semiconductor represented by the general formula below:Cu1-zIn1-xGaxSe2-ySy(wherein 0≦x≦1, 0≦y≦2, 0≦z≦1),and it is a CIS semiconductor when x=0 or a CIGS semiconductor when x>0. The CIS and CIGS are collectively described herein as “CI(G)S”.
Conventional thin-film type photoelectric conversion devices, such as CI(G)S photoelectric conversion devices, typically include a buffer layer between the photoelectric conversion layer and a transparent conductive layer (transparent electrode) formed above the photoelectric conversion layer. The buffer layer may be a CdS layer, or a ZnS layer which does not contain Cd, in view of environmental load. The buffer layer serves to achieve (1) prevention of recombination of photogenerated carrier, (2) control of band discontinuity, (3) lattice matching, (4) coverage of surface unevenness of the photoelectric conversion layer, etc. With respect to the CI(G)S photoelectric conversion devices, etc., which have relatively large surface unevenness of the photoelectric conversion layer, film formation may be achieved using CBD (Chemical Bath Deposition), which is a liquid phase process, in order to satisfy the condition (4) above.
Conventionally, it is reported that energy conversion efficiency of the photoelectric conversion layer is improved by diffusing n-type ions (if the buffer layer is made of CdS, the n-type ions are Cd2+, or if the buffer layer is zinc-based, the n-type ions are Zn2+) during formation of the buffer layer on the photoelectric conversion layer (the CBD process).
In the case where the buffer layer is formed using the CBD process, however, diffusion of the n-type ions, such as Zn2+ or Cd2+, and film formation of the buffer layer simultaneously progress. Therefore it is difficult to control both the thickness of the buffer layer and the amount of the diffused n-type ions to be optimal. It is believed that a larger amount of the diffused n-type ions results in a higher photoelectric conversion efficiency, and on the other hand, an excessively large thickness of the buffer layer results in degradation of the photoelectric conversion efficiency.
Japanese Patent No. 4320529 (hereinafter, Patent Document 1) states that, when the buffer layer is formed using the CBD process, diffusion of the Zn or Cd component and the film formation of the ZnS or CdS simultaneously progress, and this tends to cause variation in properties due to the crystal properties and the surface condition of the light-absorbing layer (the photoelectric conversion layer), and proposes a method for achieving optimal diffusion of the n-type dopant (the n-type ions) and optimal formation of the buffer layer at the same time. The method proposed in Patent Document 1 includes, for forming the buffer layer on the photoelectric conversion layer using the CBD process, a first step of diffusing the n-type dopant at an interface of the photoelectric conversion layer, a second step of forming a first buffer layer in a surface reaction rate-limited region, and a third step of forming a second buffer layer on the first buffer layer in a feed rate-limited region, thereby achieving both the optimal diffusion of the n-type dopant and the optimal formation of the buffer layer.
Further, it is taught in M. Bär et al., “Chemical insights into the Cd2+/NH3 treatment—An approach to explain the formation of Cd-compounds on Cu (In, Ga) (S, Se)2 absorbers”, Solar Energy Materials & Solar cells, Vol. 90, pp. 3151-3157, 2006 (hereinafter, Non-Patent Document 1) that, by forming the buffer layer using a gas-phase process after diffusion of the n-type ions using Cd2+/NH3 has been carried out using a liquid phase process, a higher photoelectric conversion efficiency can be achieved than that in the case where the diffusion of the n-type ions and the formation of the buffer layer are simultaneously carried out using the CBD process.
However, in the method disclosed in Patent Document 1, three processing steps are carried out from the diffusion to the formation of the buffer layer with changing the temperature to three different temperatures, and this requires complicated temperature control. Further, when the second stage of buffer layer formation is started after the first stage of diffusion, it is necessary to add a solution of a group VIb element, and this may result in a large apparatus configuration.
In the method disclosed in Non-Patent Document 1, the diffusion step is carried out in liquid phase and the buffer layer forming step is carried out in gas phase, and this inevitably results in a large apparatus configuration.
For practical application, it is desired to simplify the production process and equipment to reduce production costs.